The ability of whole cell and acellular pertussis vaccines to protect against severe pertussis has been clearly demonstrated, however a large proportion of vaccinated individuals exposed to B. pertussis become infected, and many develop a cough that lasts for weeks. The nature of the protective response is not well understood, and without this knowledge it will not be possible to improve the pertussis vaccine. The investigators propose to use functional assays to assess immunity using serum from humans with various types of exposure (infected individuals, vaccinated individuals, and negative controls). They will examine complement killing, opsonization, and phagocytosis of Bordetella pertussis, and pertussis toxin neutralization. Specific AIM 1-Antibodies that mediate complement killing. The BrkA protein of B. pertussis confers resistance to killing by complement, however some individuals produce an antibody response that is able to overcome this resistance mechanism. They will identify the targets of the bactericidal antibodies. Specific AIM 2-Antibodies that mediate opsonization and phagocytosis. The ability of human antibodies to promote opsonization and phagocytosis by macrophages, monocytes, and neutrophils will be evaluated using bacteria labeled with green fluorescent protein. They will examine the ability of virulence factors such as adenylate cyclase toxin and pertussis toxin to interfere with this process, and they will identify antibodies that promote opsonization. Specific AIM 3-Neutralization of pertussis toxin. They will evaluate the ability of human antibodies to neutralize pertussis toxin toxicity to human neutrophils. In addition, they will assess if people are more likely to generate neutralizing antibodies if immunized with genetically toxoided toxin, chemically toxoided toxin, or by natural disease.